FUEL. 159 The above equations represent the combustion of carbon and hydrogen with oxygen. This never occurs in practice, for they axe burned with air, which is a mixture of oxygen and nitrogen, the proportion by weight being 23.2 oxygen and 76.8 nitrogen, and "by volume 20.9 oxygen and 79.1 nitrogen; so that the products of combustion from burning coal are composed in great part of nitrogen. The products from hard coal and soft coal vary somewhat, because soft coal contains about 5 per cent, of hydrogen, the TABLE IX-A. Products of Combustion of Hard and Soft Coal. Hard Coal. Soft Coal. Excess Air. C0a 0 CO, 0 . Per Cent. Per Cent. Per Gent. Per tent. No excess. 21.0 0.0 19.1 0.0 10 19.1 1.9 17.3 2,0 20 17.5 3.5 15.8 3.6 80 16.1 4.8 14.5 . 4.9 40 15.0 6.0 13.5 6 1 60 14.0 . ' 6.9 12.6 7.1 60 13.0 7.8 11.7 8.0 70 12.3 8.6 11.0 8.8 80 11.7 9.3 10.4 9.5 90 11.1 9.9 9.9 10.1 100 10.5 10.5 9.4 10.6 oxidization of which produces water, and ordinarily in taking a sample of the gases this water is condensed, and does not appear in the analysis. In order to burn this hydrogen it is necessary to supply a certain quantity of air which carries nitrogen, so that the products from soft coal contain a slightly higher percentage of nitrogen and a slightly lower percentage of carbonic acid than will be obtained from hard coal. Table IX-A shows the composition of the products of combustion of hard and soft coal when burned with varying amounts of air. The first line gives the results when just sufficient air is added to completely burn the carbon and hydrogen and each succeeding line shows an additional 20 per cent, of air. An excess is necessary to insure complete combustion, the amount of excess varying with the conditions under which the coal is burned,, but it is seldom possible to have complete combustion with less than 30 per cent, excess air.